The need to reduce the cost and size of electronic equipment has led to a continuing need for smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filter element that has the potential for meeting these needs is constructed from acoustic resonators. These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure is suspended in air by supporting it around the perimeter. When an electric field is created between the two electrodes via an impressed voltage, the PZ material converts some of the electrical energy into mechanical energy in the form of sound waves. The sound waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.
At the mechanical resonance, the device appears to be an electronic resonator; hence, the device can act as a filter. The mechanical resonant frequency is that for which the half wavelength of the sound waves propagating in the device is equal to the total thickness of the device for a given phase velocity of sound in the material. Since the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. Resonators for applications in the GHz range may be constructed with physical dimensions less than 100 microns in diameter and few microns in thickness.
At the heart of Thin Film Bulk Acoustic Resonators (FBARs) and Stacked Thin Film Bulk Wave Acoustic Resonators and Filters (SBARs) is a thin sputtered piezoelectric film having a thickness on the order of one to two microns. Electrodes on top and bottom sandwich the piezoelectric acting as electrical leads to provide an electric field through the piezoelectric. The piezoelectric, in turn, converts a fraction of the electric field into a mechanical field. A time varying “stress/strain” field will form in response to a time-varying applied electric field.
To act as a resonator, the sandwiched piezoelectric film must be suspended in air to provide the air/crystal interface that traps the sound waves within the film. The device is normally fabricated on the surface of a substrate by depositing a bottom electrode, the PZ layer, and then the top electrode. Hence, an air/crystal interface is already present on the topside of the device. A second air/crystal interface must be provided on the bottom side of the device.
A publication entitled, “Bulk Acoustic Resonators and Filters for Wireless Applications,” by K. Lakin, State-of-the-Art Filter Design Using EM and Circuit Simulation Techniques, 2001 IEEE International Microwave Symp. Workshop Notes, MTT-8, Filters and Passive Components, May 20, 2001, describes the general features of thin film resonator (TFR) technology.
There are basically two types of thin film resonators (TFRs): the membrane type and the mirror type. The mirror type is also referred to as a solidly mounted resonator (SMR). The membrane type of TFR has an advantage over the SMR type in that the membrane type of TFR has a better energy confinement due to the air interface on both sides of the piezolayer. The improved energy confinement enables higher coupling coefficients that lead to larger filter bandwidth.
Unfortunately, the fabrication of these membrane structures is very complex. The challenges in the manufacturing process for these types of structures include 1) depositing multiple films with severe constraints on smoothness; 2) being able to accurately measure and monitor the thickness of each layer (especially multiple layers; 3) manufacturing a high-volume, high throughput process that is amenable to multiple depositions in one process step. This last step is important when planning for capacity; 4) finding compatible metals, piezoelectric materials, and the release material, 5) very thin (˜0.5 micrometer) AlN dielectric layer leading to electro-static breakdown problems; 6) providing robust anchor points for the suspended membranes without introducing significant acoustic energy drain (lowers device Q); 7) aligning the various layers so that there are no “ghost” FBAR segments that might resonate at slightly different frequencies. For these reasons, the state of the art for manufacturing thin film resonators has opted for the SMR structure over the membrane type of structure.
There are numerous publications that describe different aspects of the manufacture of SMR structures. For example, a publication entitled, “ZnO Based Thin Film Bulk Acoustic Wave Filters for EGSM Band” by J. Kaitila, M. Ylilammi, J. Molarius, J. Ella, and T. Makkonen, Ultrasonics Symposium 2001 describes ZnO based filters that have a solidly mounted resonator structure.
Another publication entitled, “Temperature Compensated Bulk Acoustic Thin Film Resonators” by K. M. Lakin, K. T. McCarron, and J. F. McDonald, 2000 IEEE Ultrasonics Symp. Proc., pp. 855-858, Oct. 22-25, 2000, Caribe Hilton, San Juan, Puerto Rico, describes temperature compensated TFRs that have a solidly mounted resonator structure.
Another publication entitled, “Filter Banks Implemented with Integrated Thin Film Resonators” by K. M. Lakin, K. T. McCarron, J. Belsick and R. Rose, 2000 IEEE Ultrasonics Symp. Proc., pp. 851-854, Oct. 22-25, 2000, Caribe Hilton, San Juan, Puerto Rico, describes filter banks with TFR that have a solidly mounted resonator structure.
Based on the foregoing, there remains a need for a process to manufacture acoustic resonators that overcomes the disadvantages set forth previously.